The heavy metal induction of metallothionein synthesis is a useful model system for studying eukaryotic gene regulation. Metallothionein genes from mice and humans have been cloned and characterized. The DNA sequences involved in heavy metal induction have been analyzed by in vitro mutagenesis and gene transfer techniques. Gene dosage experiments suggest that induction requires a positive transcriptional activator, and approaches to isolating this factor are being explored. Metallothionein gene sequences have also been used to develop an efficient, versatile set of mammalian cell expression vectors.